Group III-V materials such as gallium arsenide and indium phosphide are used, for example, for making lasers, light emitting diodes, microwave oscillators, and light detectors. Group H-VI materials such as cadmium sulfide may also be used for making light detectors and other devices. Most commercial use of such compound semiconductors requires the growth of large single-crystal ingots from which wafers can be cut for the subsequent fabrication of useful devices. One of the more promising methods for such crystal growth is the vertical gradient freeze (VGF) method, particularly the VGF method described in the U.S. Pat. No. 4,404,172, of W. A. Gault, granted Sep. 13, 1983, and in the paper, "A Novel Application of the Vertical Gradient Freeze Method to the Growth of High Quality III-V Crystals," by W. A. Gault et al., Journal of Crystal Growth, Vol. 74, pp. 491-506, 1986, both of which are hereby corporated herein by reference.
According to the VGF method, raw semiconductor material is placed in a vertically extending crucible including a small cylindrical seed well portion at its bottom end which snugly contains a monocrystalline seed crystal. A frustoconical transition region connects the main portion of the crucible to the cylindrical seed well. Initially, the raw material and a portion of the seed crystal are melted. The power to the system is then reduced in such a manner that solidification or freezing proceeds vertically upwardly from the seed crystal, with the crystal structure of the grown ingot corresponding to that of the seed crystal.
While the VGF method seems to work better than other methods for reducing the density of imperfections in the finished ingot, the number of imperfections in such ingots still constitutes a problem. A particularly troublesome problem is known as "twinning" in which a fault in the crystal structure separates portions of the ingot having different crystal orientations. The prior art contains many references to the problem of twinning and to various methods for alleviating this problem. Nevertheless, there continues to be a long-felt need in the industry for methods for growing large single crystals of compound semiconductor material with fewer defects and imperfections and for growing such crystals reliably and at lower cost.